Revert "fix configure script to check pthread_atfork(3) in the correct way"

This reverts commit 462e13b1.

folly/configure.ac

@@ -264,23 +264,6 @@ AC_SEARCH_LIBS([clock_gettime], [rt],
     [Define to 1 if we support clock_gettime(2).]),
   [])
 
-# Check for pthread_atfork(3). This is not in an AC_CHECK_FUNCS() because we
-# want to include pthread.h if necessary.
-AC_CACHE_CHECK(
-  [for pthread_atfork support],
-  [folly_cv_prog_cc_pthread_atfork],
-  [AC_COMPILE_IFELSE(
-    [AC_LANG_SOURCE[
-      #include <pthread.h>
-      void func() {pthread_atfork(NULL, NULL, NULL);}]
-    ],
-    [folly_cv_prog_cc_pthread_atfork=yes],
-    [folly_cv_prog_cc_pthread_atfork=no])])
-
-if test "$folly_cv_prog_cc_pthread_atfork" = "yes"; then
-  AC_DEFINE([HAVE_PTHREAD_ATFORK], [1], [Define to 1 if the compiler supports pthread_atfork])
-fi
-
 # Checks for library functions.
 AC_CHECK_FUNCS([getdelim \
                 gettimeofday \
@@ -289,6 +272,7 @@ AC_CHECK_FUNCS([getdelim \
                 pow \
                 strerror \
                 pthread_yield \
+                pthread_atfork \
                 malloc_size \
                 malloc_usable_size \
                 memrchr \